Natural wetland systems have often been described as the "earth's kidneys" because
they filter pollutants from water that flows through on its way to receiving lakes,
streams and oceans. Because these systems can improve water quality, engineers
and scientists construct systems  that replicate the functions of natural
wetlands. Constructed wetlands are treatment systems that use natural
processes involving wetland vegetation, soils, and their associated microbial
assemblages to improve water quality.
How do treatment wetlands
      Natural wetlands perform many functions
       that are beneficial to both humans and
wildlife. One of their most important functions
is water filtration. As water flows through a
wetland, it slows down and many of the
suspended solids become trapped by vegetation
and settle out. Other pollutants are transformed
to less soluble forms taken up by plants or
become inactive. Wetland plants also foster the
necessary conditions for microorganisms to live
there. Through a series of complex processes,
these  microrganisms also transform and remove
pollutants from the water.

Nutrients, such as nitrogen and phosphorous, are
deposited into wetlands from stormwater runoff,
from areas where fertilizers or manure have been
applied and  from leaking septic fields. These
excess nutrients are often absorbed by wetland
soils and taken up by plants and microorganisms.
For example, wetland microbes can convert
organic nitrogen into useable, inorganic forms
(NO3 and NH,,) that are necessary for plant
growth and into gasses that escape to the

Why build them?
Wetlands are some of the most biologically
diverse and productive natural ecosystems in the
world.  While not all  constructed  wetlands
replicate  natural ones, it makes sense to
construct wetlands that improve water quality
and support wildlife habitat. Constructed
wetlands  can also be  a cost-effective and
technically feasible approach to treating
wastewater. Wetlands are often less expensive to
build than traditional wastewater  treatment
options, have low operating and maintenance
expenses  and can handle fluctuating water levels.
Additionally, they are aesthetically pleasing and
can reduce or eliminate odors associated with
                            A Popular Idea
                         Designing and building
                         wetlands to treat
                         wastewater is not a new
                         concept. As many as
                         5,000 constructed
                         wetlands have been built
                         in Europe and about
                         1,000 are currently in
                         operation in the United
                         States. Constructed
                         treatment wetlands, in
                         some cases involving the
                         maintenance of
                         important wetland
                         habitat, have become
                         particularly popular in
                         the Southwest, where the
                         arid climate makes the
                         wetland habitat
                         supported by these
                         projects an especially
                         precious resource.
                                  Wetland Plants
      Gravel Substrate
             Impermeable Liner
  Water Level Control
Wetland plants and associated microorganisms treat wastewater as it flows
through a constructed wetland system.
How are they  built?
Constructed wetlands are generally built on
uplands and outside floodplains or floodways in
order to avoid damage to natural wetlands and
other aquatic resources. Wetlands are frequently
constructed by excavating, backfilling, grading,
diking and installing water control structures to
establish desired hydraulic flow patterns. If the
site has highly permeable soils, an impervious,
compacted clay liner is  usually installed and the
original soil placed over the liner.  Wetland
vegetation is then planted or allowed to establish

Design and Planning
If planned and maintained properly, treatment
wetlands can provide wastewater treatment and
also promote water reuse, wildlife habitat, and
public use benefits.  Potentially harmful
environmental impacts, such as the alteration of
natural hydrology, introduction of invasive
species and the disruption of natural plant and
animal communities can be avoided by
following proper planning, design, construction
and operating techniques. The following
guidelines can help ensure a successful project:

   Construct treatment wetlands, as a rule,  on
    uplands and outside floodplains in order to
    avoid damage to natural wetlands and other
    aquatic resources, unless pretreated effluent
    can be used to restore degraded systems.

   Consider the role of treatment wetlands
    within the watershed (e.g., potential water
    quality impacts, surrounding land uses and
    relation to local wildlife corridors).

   Closely examine site-specific factors, such
    as soil suitability, hydrology, vegetation, anc
    presence of endangered species or critical
    habitat,  when determining an appropriate
    location for the  project in order to avoid
    unintended  consequences, such as
    bioaccumulation or destruction of critical

   Use water control measures that will allow
    easy response to changes in water quantity,
    quality, depth and flow.

    Create and follow a long-term
    management plan that includes regular
    inspections, monitoring and maintenance.
This hog operation in Taylor County, Iowa, uses a wetland system constructed
on a series of hillside terraces to filter and purify wastewater. Water quality
tests  indicated that the effluent from the treatment wetland was cleaner than
that required for wastewater treatment plants.

Tras t3iixs  Projaol  Irnprovos Writer  QnnlHy
 In 1990, city managers in Phoenix, Arizona, needed to improve the performance of
 their 91st Avenue Wastewater Treatment Plant to meet new water quality standards
 issued by the Arizona Department of Environmental Quality. After learning that
 upgrading  their treatment plant might cost as much as $635 million, the managers
 started to look for a more cost-effective way to polish the treatment plant's wastewater
 discharge into the Salt River. A preliminary study suggested that the city consider a
 constructed wetland system that would polish effluent, while supporting high-quality
 wetland habitat for migratory waterfowl and shorebirds, including endangered species,
 and protecting downstream residents from flooding at a lower cost than retrofitting
 their existing treatment plant. As a result, the 12-acre Tres Rios Demonstration Project
 began in 1993 with assistance from the U.S. Army Corps of Engineers, the Bureau of
 Reclamation and EPA's Environmental Technology Initiative and now receives about
 two million gallons of effluent per day. The demonstration project was so successful
 that the city and the Bureau of Reclamation asked EPA for help in expanding the
 project to a full-scale, 800-acre project. For more information on the Tres  Rios
 Constructed Wetlands Project, visit,
                                                                                                     EPA 843-F-03-013
                                                                                                        Office of Water
                                                                                                          August 2004
 Treatment Wetlands (2004), Robert H. Kadlec and Robert L. Knight, Lewis Publishers, Boca Raton, Fl.

 Guiding Principles for Constructed Treatment Wetlands: Providing for Water Quality and Wildlife Habitat (2000), United
   States Environmental Protection Agency, EPA 843-B-00-003. Available online at
   constructed/guide, html

 Constructed Wetlands Handbooks (Volumes 1-5): A Guide to Creating Wetlands for Agricultural Wastewater, Domestic
   Wastewater, Coal Mine Drainage and Stormwater in the Mid-Atlantic Region (1993-2000), United States Environmental
   Protection Agency.  Available online at

 Handbook for Restoring Tidal Wetlands (2000), Joy B. Zedler, CRC Press, Boca Raton, FL.